Lecture 11: PPP (Pentose Phosphate Pathway)

What is the function of the PPP?

Provide NADPH primarily for biosynthesis and for lessening oxidative stress

What are the two phases in the cytoplasm?

Oxidative & Nonoxidative

The Oxidative phase?

Irreversible phase to generate NADPH

( G6P + 2NADP+ + H2O→ Ru5P+ CO2 + 2 NADPH + H+ )

The Nonoxidative phase? 

Reversible phase to recycle R5P 

3 Ru5P 2⇌ 2 F6P + 1 GA3P

What is Phase 1?

Oxidative decarboxylation by G6P dehydrogenase
G6P + 2NADP+ + H2O→ Ru5P+ CO2 + 2 NADPH + H+
•Ru5P is 5 carbon ketose

What is Phase 2?

•Non-oxidative
•Carried out by isomerase, transaldolase and transketolase

What is step 1 of Phase 2?

Ru5P to R5P and Xu5P
•Ru5P to R5P by phosphopentose isomerase
•Ru5P to Xu5P by phoshopentose epimerase
•R5P is used to synthesize RNA/DNA, FAD/NAD+ and CoA
•The excessive Ru5P, R5P and Xu5 is recycled to F6P +GAP

What is step 2 of Phase 2?

R5P + Xu5P → GAP + Su7P
•By transketolase
•Su7P is 7-carbon ketose

What is step 3 of Phase 2?

GA3P + Su7P → F6P + E4P
•By transaldose
•E4P is an aldose

What is step 4 of Phase 2?

E4P + Xu5P → F6P + GA3P
•By transketolase

How is PPP Regulated?

Using:

ATP 

NADPH 

R5P

1 of PPP Regulation?

Only need R5P
Glycolysis step 1-5 follwed by PPP phase II:
G6P → 2F6P + 1GAP → 3 R5P

2 of PPP Regulation?

Need both R5P and NADPH
PPP phase I:
G6P +2NADP+ → R5P+ CO2 + 2 NADPH

3 of PPP Regulation?

Only need NADPH
PPP phase I: G6P +2NADP +→ R5P+ CO2 + 2 NADPH
PPP phase II: 3R5P → 2F6P + 1GAP
Gluconeogenesis: 2F6P + 1GAP → G6P

4 of PPP Regulation?

Need NADPH and ATP
PPP phase I: G6P +2NADP+ — R5P+ CO2 + 2 NADPH
PPP phase II: 3R5P — 2F6P + 1GAP
Glycolysis: 3-10 for F6P or 6-10 for GAP followed by PDC and CAC

What is primarily used in Biosynthesis?

NADPH

What does NADPH do?

• Antagonizes Oxidative stress 

• Reactive oxgen species (ROS) inflict harm on cell, leading to cell death

• Glutathione (GSH) is a tripeptide with a SH group, which is able to reduce OS, detoxicating it and generating GSSG.

• GSSG is reduced to GSH by NADPH